A series of single-phase metal oxide photocatalysts K4Ce2M10O30 (M,= Ta, Nb), capable of evolving H-2 and O-2 from aqueous solutions containing a sacrificial electron donor (Na2SO3) and acceptor (AgNO3), respectively, under visible light irradiation (lambda > 420 nm) without any cocatalyst were presented. The activities were greatly enhanced by the incorporation of Pt, RuO2 and NiO (NiOx) as co-catalysts on the prepared oxides. The photocatalysts have an appropriate band gap energy ca. 1.8-2.3 eV (corresponding to absorption edge of 540-690 nm) and excellent chemical potential level for utilization of solar energy, representing candidates of photocatalysts for hydrogen evolution from water decomposition. Density function theory (DFT) calculation indicated that while their valence bands are composed of hybridization with O 2p + Ta 5d (or Nb 4d) and occupied Cc 4f orbitals, the conduction bands of these photocatalysts K4Ce2M10O30 (M = Ta, Nb) are mainly attributable to the Ta 5d (or Nb 4d) orbitals. Although the unoccupied Cc 4f orbitals have overlap in the bottom of conduction band, they are less effective in transferring electrons and photocatalytic activities for their high localized nature. The contribution of these orbitals to the energy bands affects the electronic structure of the both photocatalysts and gives rise to their differences in light absorption and photocatalytic activities. (c) 2006 Elsevier B.V. All rights reserved.